I've had joint failures on the 1st two tanks that I built, but I avoided disaster because I noticed some crazing early on and monitored it closely. When the joints started to fail, I just moved my fish, and retired the tanks. In both cases, I was probably lucky that there was no critical failure. Both of these tanks used the minimum thickness recommended and they only lasted about 2 years each.
I like to save a buck as much as the next guy, but when it comes to big tanks I think you have to consider the long-term costs of going the cheap route because the tank will start wearing down after a year or 2. If you do it right the 1st time, the tank will last 20 years. The money spent on the extra glass thickness will be the best investment you ever made.